In the known devices for measuring the concentration of gases by radiation absorption at characteristic absorption wave lengths for the gas, the gas to be analysed is introduced into a measuring chamber. This is subjected to radiation of the specific wave length which enters the measuring chamber with a flux .phi..sub.o. This flux is weakened if gas molecules having a significant absorption characteristic at that wave-length are present and if present, the radiation leaves the measuring chamber with a reduced flux .phi..
According to Lambert-Beer's law, the relationship is written: EQU .phi.=.phi..sub.o e.sup.-mlc
Where
M is a material constant, PA1 L is the length of the radiation path in the absorbing medium, and PA1 C is the concentration of the absorbing gas in the measuring chamber.
If it is desired to measure very small concentrations, then, for a minimum weakening ratio which is given by the relationship .phi./.phi..sub.o and is limited by the resolution and sensitivity of the detectors and amplifiers, this is only possible by increasing the path length l of the radiation through the chamber.
In known spectrophotometric devices for gas analysis, measuring chambers are used in which the radiation path is bent via an optical system. For example, a principle given by White makes it possible to produce path lengths of up to ten meters. However, the apertures are small and the volume of the chamber amounts to more than six liters. For measuring the concentration of alcohol molecules in exhaled breath, the measuring chamber must have a very small volume in order to ensure that only air from deep in the lungs fills the measuring chamber. For this reason the chamber volume must not exceed 100 cm.sup.3.
A device is described in U.S. Pat. No. 3,319,071 in which a sphere with highly reflective inner walls forms the measuring chamber. However, this arrangement is quite unsuitable for measuring the concentration of alcohol in the breath, as a sphere has the greatest volume with the smallest outer dimensions and exactly the opposite is desired.